McEwan won for three articles - on food production at Paignton Zoo, the renovation of Chiswick Park and green roofs and walls.
Judges described the articles as covering "three distinctive and culturally diverse topics in a balanced and informative way". Writers for HW and sister publication Garden Retail have claimed the award in each of the past five years.
HW deputy editor Matthew Appleby and writer Maureen Keepin were among the finalists, while awards for consumer gardening journalism went to Amateur Gardening news editor Marc Rosenberg, Kitchen Garden's Joe Maiden and Victoria Summerley of The Independent. The Landscape Man took garden TV programme of the year.